The world resources of oil exist in a number of geological formations with more than 40% of the reservoirs formed in carbonates e.g. limestone (CaCO3) or dolomite CaMg(CO3)2. From these formations oil is recovered by drilling and pumping. Also oil-sand and oil-shale reservoirs account for a significant portion of the world's combined oil-resources.
The oil in rock formations in general is present in pores and cavities of the rock, sand or shale. The accessibility to the oil in an oil field is largely determined by the porosity of the reservoir formation and the permeability of the oil, both factors which can vary a lot depending on location and whether the reservoir drilled contains a significant number of cracks and fractures at the drill location. Typically such oil-bearing formations are found beneath the upper strata of the earth, referred to generally as the overburden, at depths of 300 meters or more, whereas oil in sand and shale can be found already at depths of 20 meters and below.
Inside such oil bearing rock formations, the oil is detained within the pores primarily by capillary forces, e.g. by wetting the rock surfaces, and electrostatic forces. E.g. in carbonate rock some oils are oxidized to carboxylic acids which further enhances the electrostatically binding to the positively charged carbonate rock. Often, however, the rock surfaces are also wetted by water, which leads to complicated water-oil interactions inside the rock formation.
In oil recovery a pressure must be added which is sufficient to exceed the electrostatic and the capillary binding forces of the oil to the rock in order to achieve an oil flow. During production, oil will be recovered from the larger pores first, which are then filled with injection water, which are injected into the drill hole at pressures of several hundred bar in order to effect an oil migration. This leads to an increased water/oil ratio during recovery.
If the capillary forces or the electrostatic forces binding the oil can be reduced, it has long been recognized that a higher recovery can be obtained. Various methods of altering the wetting properties of oil on carbonate surfaces have been suggested and implemented in the prior art, in particular injection of surfactants and negatively charged counter-ions to disrupt wetting and electrostatic association of oil and rock. Also viscosity reducing methods, notably heat, have been systematically used in oil production.
Electrically enhanced oil production (EEOP) is a proven quaternary oil recovery (QOR) technology and has been shown to be economically viable at recovery costs below other methods of recovery, such as e.g. secondary and tertiary oil recovery technologies.
Most methods of electrically enhanced oil-production (EEOP) involve passing direct current (DC) between cathodes in producing well completion intervals and anodes either at the surface and/or at depth in other wells. The electrokinetic mechanisms indicated to be operative based on the available data of the prior art are 1) joule heating, 2) electro-chemical reactions and 3) electro-osmotic flow (EOF). In general, however, the physico-chemical processes observed during EEOP are co-existing and all contribute to the beneficial results on oil recovery from the method. It has been established in many field test that EEOP as a quaternary oil recovery (QOR) technique is superimposable on existing secondary and tertiary recovery techniques without limitations.
A representative method for enhanced oil recovery from carbonate reservoirs is described in US 2013/0277046 A1, the contents of which is hereby incorporated by reference; the method comprising the steps of selecting an underground formation comprising an oil-bearing carbonate reservoir, positioning two or more electrically conductive elements at spaced apart locations in proximity to said formation, at least one of said conductive elements being disposed in or adjacent to a borehole affording fluid communication between the interior of said borehole and said formation, passing a controlled amount of electric current along an electrically conductive path through said formation, said electric current being produced by a DC source including a cathode connected to another of said conductive elements, said electrically conductive path comprising at least one of connate formation water and an aqueous electrolyte introduced into said formation, and withdrawing oil from at least one of said boreholes.
A drawback of the currently known methods in the art is a requirement of high electrical potentials between the electrodes of the EEOP, preferably not less than 0.4 V per running meter between electrodes, resulting in increased energy consumption during oil recovery. Also the methods of the prior art have failed to be efficient in viscous or heavy oil reserves.
Surprisingly, the present inventors have now discovered that the energy requirement can be significantly lowered compared to conventional methods of EEOP by following the methods as described in the present invention, while at the same time reducing oil-viscosity and allowing oil recovery from hard oil reserves.